This thesis aimed to assess the ability of a self-paced CPET protocol to determine patient
fitness prior to major elective surgery and predict postoperative outcomes. A number of
limitations have been highlighted in regard to the general nature of sCPET protocols
including: the variable and uncertain test duration, no clear end point to work towards
and, individuals having no control over their work rate (Noakes 2008). As CPET is an
important component of a patient perioperative care (West et al., 2011), alternative test
protocols should be considered if they improve clinical decision-making, the patient
experience, and/or general working practice for clinicians. The SPV protocol may provide
a viable alternative to sCPET protocols in clinical practice because it ensures all patients
exercise for the recommended 10 minutes which will reduce the risk of obtaining invalid
CPET data, it takes away the need of clinicians having to choose the most appropriate
work rate increments to ensure a valid test, and it also allows patients to have full control
over the test and chosen work rates.
The first experimental study of this thesis (Chapter 4) demonstrated the SPV to be a
reliable indicator of CPET derived variables in young, healthy participants. Prior to this,
there was limited information about the reliability of the SPV. The results from Chapter
4 established the CV for VO2peak (4.7%) to besimilar to that found by Straub et al. (2014),
and also previous findings using sCPET protocols (Froelicher et al., 1974; Janicki et al.,
1990). This is important as good test-retest is essential when conducting CPET in clinical
environments as often there is only enough time in some clinical pathways for one test to
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patient fitness without the need for familiarisation of the protocol is therefore essential.
However, it is also important that the test provides a true reflection of a patient’s
cardiorespiratory fitness. As a consequence Chapter 5 compared the physiological
responses from the SPV to a sCPET protocol in both young and old healthy populations.
All previous studies investigating the SPV have been conducted on younger individuals,
whereas clinical populations tend to be of middle to old age. Results from Chapter 5
demonstrated that young participants achieved a significantly higher VO2peak in the SPV
vs. the sCPET, which is comparable to previous studies (Astorino et al., 2015; Mauger &
Sculthorpe 2012; Mauger et al., 2013). In addition to the higher VO2peak, the SPV also
produced a significantly higher peak Q, SV, VE, RER and PO, with a trend for higher
end-exercise lactate values (P = 0.06). These findings suggest that a higher physiological
work rate was achieved in the SPV compared to the sCPET. In particular, it has previously
been established that Q is strongly associated with VO2, and is a principal limiting factor
for VO2peak (Bassett & Howley 2000). Therefore as suggested by Astorino and colleagues
(2015), the enhanced Q response is likely to be the key reason for the increased VO2peak.
In the older adult group there were no significant differences in VO2peak or peak Q
between test protocols. This again may supports the notion that Q is a key contributor to VO2peak. However, interestingly similar to the young group, the older adult SPV data
showed significantly higher peak RER and PO values, with a statistical trend for a higher
end-exercise lactate (P = 0.05) compared to the sCPET. These findings suggested that
despite the older group reaching a higher work load and metabolic rate in the SPV, Q did
not appear to respond in the same way as in the young group. The reasons for this are unclear, although the lack of difference in Q may be a result of the normal age-related
changes that occur in cardiac function (Lakatta & Levy 2003), preventing Q to increase beyond a particular point. Nevertheless, the findings from Chapter 5, combined with
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previous research (Astorino et al., 2015; Chidnok et al., 2013; Faulkner et al., 2015;
Mauger and Sculthorpe 2012; Mauger et al., 2013; Straub et al., 2014), provides further
evidence that the SPV is able to produce similar or superior levels of CPET derived
variables. The other important finding from the Chapter 5 study was that the SPV could
be safely used in an older population, reflective of the general ages of those patients
needing preoperative CPET. Rather than testing a clinical population first, it was deemed
safer to use a healthy older adult population as some have suggested the SPV might
provide heightened risks for older clinical populations (Eston, Crockett & Jones 2014).
The principle aim of Chapter 6 was to assess the utility of the SPV in a clinical population.
Specifically, the study aimed to determine the validity and reliability of the SPV protocol
for assessing cardiopulmonary fitness in a stable clinical population (early post-MI patients). Results demonstrated the SPV to reliably produce higher VO2peak values, along
with significantly higher peak HR, VE and PO values. This is surprising as the older adult
group in Chapter 5 demonstrated similar VO2peak values between SPV and sCPET
protocols, even though mean ages were similar between participants (Chapter 5: 59 ± 6 years; Chapter 6: 58 ± 8 years VO2peak). A possible explanation might be provided by
differences in peak PO between the sCPET and SPV between the two studies; there was
a greater difference between SPV and sCPET protocols in the post-MI patients (~35 W)
compared to the healthy older population (~19 W). This greater difference in PO between
protocols in the post-MI patients is a likely reason why they also display a significantly higher VO2peak in study 3 (Chapter 6), and may also be why there is a lack of difference
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The primary aim of Chapter 7 was to assess the ability of the SPV protocol to predict
postoperative outcomes in the preoperative assessment of patients having elective
surgery, compared to a sCPET protocol. In contrast to previous research (Junejo et al.,
2012; West et al., 2014; West et al., 2016), none of the CPET derived variables from
either protocol were associated with postoperative morbidity. It is likely this is a result of
the low sample size and wide range of surgery types used in the study. Due to the small
sample size for morbidity cases, statistical power was increased by combining 3 and 5
day POMS census points. Resultant logistic regression analysis demonstrated that oxygen
pulse at AT obtained from the SPV was the only variable to be associated with
postoperative morbidity. In addition, ROC curve analysis suggested an optimal cut-off
point of 8.5 ml beat-1, which is comparable to previous research (West et al., 2016). The
reasons for this finding, and why oxygen pulse at AT was only significant from the SPV
protocol is unclear. Additional research would be needed to investigate this further.
As with earlier chapters in this thesis, results from Chapter 7 demonstrated the SPV
produced a significantly higher VO2peak, VE, HR and peak PO values when compared to
the sCPET. However, it is acknowledged that the average difference of ~1 ml·kg-1· min-1
from chapter 7 and ~2 ml·kg-1·min-1 from Chapter 6 is not likely to be a clinically
meaningful difference. To support this, the study outlined in Chapter 7 found no
differences in the Carlisle 30-day mortality score (Carlisle et al., 2015), which suggests that the observed difference in VO2peak would not influence risk stratification and thus
clinical decision making. Therefore, it can be concluded from that the SPV has the
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Previous research has suggested that the SPV may reduce the physiological demand of an
incremental exercise test in comparison to a sCPET (Lander et al., 2009); a possible
reason for the higher VO2peak being achieved by participants. However, data from the
current thesis suggests the contrary; data from the three experimental chapters that
compared physiological parameters between SPV and sCPET protocols (Chapter 5, 6 &
7), all suggest that there is an enhanced physiological workload achieved in the SPV.
Specifically, a higher HR, VE, and RER was found in all three comparative studies
(Chapter 5, 6 &7), and higher Q, SV and blood lactate seen in study 2 (Chapter 5).
Therefore, it seems that the SPV simply provides individuals with the opportunity to work
at high work rates which in turn drive VO2 to a greater level. Even though the SPV is
completely self-paced, participants were generally willing to work at higher intensities
than seen in the sCPET. This could be a result of the defined test duration and the
participants knowing the test was going to end within a defined time period may have
motivated them to work harder in the final stage. It is also possible that the continuous
fluctuation in PO and adequate pacing in the earlier stages of the test may have
contributed in minimising early fatigue, leading to a greater work rate being achieved in
the final stage of the SPV.
Throughout this thesis there has been a reoccurring discussion concerning optimal CPET
test duration. Current guidelines suggest that a CPET should last between 8-12 minutes
(American Thoracic Society/American College of Chest Physicians 2003), which was
based on the work by Buchfuhrer et al. (1983). For this test duration to be achieved, the
test administrator must select the most appropriate starting intensity and work rate
increments to ensure exhaustion within the 8-12 min period to prevent obtaining invalid
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may not be appropriate in all populations. Specifically, in studies 2, 3 and 4 (Chapter 5,
6 & 7) sub-analysis was completed to consider individuals who completed the sCPET in the ‘recommended’ time of 8-12 min, compared to those who did not (> 8 min; > 12 min). In the young healthy group from study 2, there was a higher VO2peak achieved in the SPV
(compared to the sCPET) by participants who completed the sCPET outside of the 8-12 min (> 8 min; > 12 min). There was no difference in VO2peak for those who completed the
sCPET within the 8-12 min time period, supporting findings of previous work
(Buchfuhrer et al., 1983). However, it is important to note that only 3 participants completed the test in < 8 min. Interestingly, in both clinical studies (Chapter 5 & 6) VO2peak was higher in the SPV compared to the sCPET for patients who completed the
sCPET in < 8 min and 8-12 min. There were only four patients from study 3, and two patients from study 4, who completed the sCPET in > 12 min but, VO2peak was higher in
the sCPET in all individuals. These findings support previous research (Agostoni et al.,
2005), in suggestting longer CPET durations might be beneficial for clinical populations.
However, further research is required to establish optimal CPET durations for a variety
of different populations (Midgley et al., 2008). Selecting the most appropriate work rate
increments for each individual is essential to ensure that valid CPET data is obtained. If
selected work rates are too hard for the patient they are likely to stop early giving invalid
and limited CPET information. Conversely, if the increments are too low, the patient may
not drive VO2 sufficiently to obtain a true VO2peak prior to the development of significant
peripheral fatigue. The SPV provides all patients the opportunity to exercise for 10 min,
therefore increasing the likelihood of obtaining more valid and useable CPET data.
There are potential benefits associated with using the SPV in a clinical environment. For
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may reduce the risk of having unusable data from tests that do not last long enough. If
unusable data is obtained then the test may need to be repeated, or have patient
preoperative risk being calculated using traditional, less direct methods which may under-
or over-predict risk. In turn this may increase costs associated with postoperative care
needed for that patient. The fixed test duration is also likely to improve the efficiency of
busy CPET clinics and ensure allocated appointment timeslots are adhered to. From a
clinician’s perspective, the SPV removes the need to estimate the most appropriate
starting intensity and increment rates, which will ensure a maximal stress is applied to the
cardiorespiratory system within the 10 min test duration. From a patient perspective, the
SPV provides an individual with control over the exercise intensity, so that they can freely
regulate their work rate throughout the test. The patient also has knowledge of test
duration which may help motivate them to complete the full 10 min duration of the test.
Finally, by providing useable and valid CPET data the SPV will ensure that the most
appropriate perioperative care is provided for the patient, which will ultimately assist in
improving their postoperative outcome.